The present invention relates to the formation of a borophosphosilicate glass ("BPSG") layer during the fabrication of integrated circuits on semiconductor wafers. More particularly, the present invention relates to a method for improving the reflow characteristics of a BPSG film enabling the film to fill gaps having higher aspect ratios and smaller widths while meeting the thermal budget requirements of modern day manufacturing processes.
Borophosphosilicate glass ("BPSG") has found wide use in the semiconductor industry as a separation layer between the polysilicon gate/interconnect layer and the first metal layer of MOS transistors. Such a separation layer is often referred to as premetal dielectric (PMD) layer because it is deposited before any of the metal layers in a multilevel metal structure and is used to electrically isolate portions of the first deposited metal layer from the semiconductor substrate.
In addition to having a low dielectric constant, low stress and good adhesion properties, it is important for PMD layers to have good planarization and gap-fill characteristics. BPSG deposition methods have been developed to meet these characteristics and often include planarizing the layer by heating the layer above its reflow temperature so that it flows as a liquid. The reflow process enables the BPSG to better fill high-aspect ratio, small-width trenches and results in a flat upper surface upon cooling. The heating necessary to reflow a BPSG layer can be achieved using either a rapid thermal pulse (RTP) method or a conventional furnace in either a dry (e.g., N.sub.2 or O.sub.2) or wet (e.g., steam H.sub.2 /O.sub.2) ambient. These processes are generally considered to be somewhat equivalent and thus interchangeable for many applications. If any particular benefits are attributable to one process over the others, however, persons of skill in the art generally believe that annealing a BPSG layer in a conventional furnace having a wet (steam) ambient provides better gap-fill properties than using RTP methods and that dry conventional furnace anneals are basically equivalent to RTP methods in terms of gap-fill characteristics.
Standard BPSG films are formed by introducing a phosphorus-containing source and a boron-containing source into a processing chamber along with the silicon-and oxygen-containing sources normally required to form a silicon oxide layer. Examples of phosphorus-containing sources include triethylphosphate (TEPO), triethylphosphite (TEP.sub.i), trimethylphosphate (TMOP), trimethylphosphite (TMP.sub.i), and similar compounds. Examples of boron-containing sources include trietbylborate (TEB), trimethylborate (TMB), and similar compounds.
As semiconductor design has advanced, the feature size of the semiconductor devices has dramatically decreased. Many integrated circuits (ICs) now have features, such as traces or trenches that are significantly less than a micron across. While the reduction in feature size has allowed higher device density, more complex circuits, lower operating power consumption, and lower cost, the smaller geometries have also given rise to new problems, or have resurrected problems that were once solved for larger geometries.
One example of a manufacturing challenge presented by submicron devices is the ability to completely fill a narrow trench in a void-free manner while keeping the thermal budget of the trench-filling process at a minimum. For example, in order to meet the manufacturing requirements of 0.18 micron geometry devices and below, a BPSG layer may be required to fill 0.1 micron wide gaps and narrower having an aspect ratio of up to 6:1. At the same time, these manufacturing requirements require that the thermal budget of the BPSG deposition and reflow step be kept to a minimum.
One method that manufacturers have developed in efforts to meet these and/or similar requirements is the addition of fluorine or similar halogen element to the BPSG film. Such fluorine-doped BPSG films are often referred to as "fluorinated-BPSG" or "FBPSG." Fluorine is believed to lower the viscosity of the BPSG film so that it reflows easier during the reflow step. In this manner, the addition of fluorine can be used to improve the gap-fill and planarization of BPSG layers when deposited and reflowed at a given temperature. Alternatively, the addition of fluorine can be used to reduce the reflow temperature of the BPSG film while retaining gap-fill and planarization characteristics of a BPSG film reflowed at a higher temperature. U.S. Pat. No. 5,633,211 illustrates one example of a method used to deposit a FBPSG layer.